Purpose Flexible ureteroscopes smaller than 9Fr are widely used in endourology. We systematically evaluated the functional durability of these instruments in the clinical setting. Materials and Methods We performed ureteronephroscopy 92 consecutive times in 84 patients at our hospital using a flexible Storz model 11274AA, ‡ ‡ Karl Storz Endoscopy, Tuttlingen, Germany. Circon-ACMI model AUR-7, § § Circon Corp., Santa Barbara, California. Wolf model 7325.172 ∥ ∥ Richard Wolf GmbH, Knittlingen, Germany. and Olympus model URF/P3 ureteroscope ¶ ¶ Olympus Optical Co., Melville, New York. . Preoperatively and postoperatively we evaluated all flexible ureteroscopes for luminosity, irrigant flow at 100 mm. Hg, number of broken image fibers and active deflection range. During the procedure a record was kept of the duration that the endoscope remained in the urinary tract, average irrigation pressure, method of insertion, various devices used within the working channel, need for lower pole access, and surgeon overall impression of visibility and maneuverability. Results The luminosity and irrigant flow of all endoscopes remained relatively unchanged during consecutive applications, while active deflection deteriorated 2% to 28%. Endoscopes were used for an average of 3 to 13 hours before they needed repair. The most fragile part of these instruments was the deflection unit. Conclusions Small diameter flexible ureteroscopes are effective for diagnosing and treating upper urinary tract pathology but improved durability is required. Currently they represent a highly effective but high maintenance means of achieving retrograde access to the ureter and kidney with a need for repair after only 6 to 15 uses.